Known in the art is a directly liquid cooled rotor for an electrical machine (cf. British Pat. No. 1,416,765), comprising a non-salient pole rotor body and a winding consisting of coils arranged within the respective slots formed in the rotor body and electrically connected mutually in series. Each one of the coils is made of hollow conductors to form a hydraulic loop, the cooling liquid being fed therethrough. In each of the hydraulic loops its lower or radially inner lead serves as an inlet for supplying the cooling liquid into the hydraulic loop, while its upper or radially outer lead serves as an outlet for draining the cooling liquid from said loop. Thus, the conductor for draining the cooling liquid is positioned at a greater distance from the axis of the rotor than the conductor lead for supplying the cooling liquid. The conductors for supplying and draining the cooling liquid are in communication with a cooling liquid supply line via metal tubes arranged coaxially with respect to said conductors, all the hydraulic loops being thus connected mutually in parallel.
It is common knoledge that the rotor cooling depends upon the flow rate of the cooling liquid passing through the hydraulic loops. The flow rate of the cooling liquid in the hydraulic loop is a function of the head of the liquid and of the resistance of the hydraulic loop to the liquid flow, the head depending upon the difference between the draining level and the supply level with respect to the rotor axis, while the resistance to the cooling liquid flow is defined in general by the cross-sectional area of the cooling ducts and their total length. Inasmuch as in the prior art rotor the cooling liquid is supplied into the hydraulic loops at the same distance from the rotor axis and is drained also at the same distance from the rotor axis as well, the circulation of the cooling liquid in all the hydraulic loops is effected with the same head. The flow rate of the liquid, however, can be different in the symmetrically arranged hydraulic loops, resulting from deviations in diameter of the hydraulic loop ducts, different distances between the coils and the rotor surface, and different thicknesses of coil conductor insulators. All this leads to a non-symmetrical cooling of the rotor and to thermal unbalance depending upon the load of the electrical machine. As a result, the axis of rotation of the rotor displaces from its nominal position due to added thermal deformations, and the rotor shaft vibrates in its bearings, this substantially decreasing the reliability of the electrical machine. Under variable load conditions of the electrical machine operation the thermal unbalance cannot be eliminated by using balance weights fixed on the rotor.